From LoveToKnow 1911

KOCH, a tribe of north-eastern India, which has given its name to the state of
Kuch Behar. They are
probably of Mongolian stock, akin to the Mech, Kachari, Garo and Tippera tribes, and originally
spoke, like these, a language of the Bodo group. But since one of their chiefs
established a powerful kingdom at Kuch Behar in the 16th century they have gradually
become Hinduized, and now adopt the name of Rajbansi (= "of royal
blood"). In 1901 the number in Eastern Bengal and Assam was
returned at nearly 21 millions.